pH scale of acidity and alkalinity, acids, bases-alkalis, salts and neutralisation

1. Examples of everyday acid-alkali chemistry in the home or industry

The chemistry of acids and bases-alkalis is introduced by looking at common domestic examples in the home and not just in industry or the chemical laboratory. Lime, antacids, lime, bee/wasp stings, sodium bicarbonate, ammonia, sodium hydroxide, hydrochloric acid all get a mention!

GCSE/IGCSE Sub-index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of acid-alkali chemistry : 2. pH scale, indicators, ionic theory of acids-alkali neutralisation : 3. pH examples of acid, neutral or alkaline solutions : 4. Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions : 5. Reactions of bases-alkalis like sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation : 8. Important formulae, salt solubility and water of crystallisation : 9. Further examples of word/symbol equations for salt preparations : 10. More on Acid-Base Theory and Weak and Strong Acids

In the HOME: Alkaline lime (CaO, calcium oxide) or slaked lime (Ca(OH)₂, calcium hydroxide), are put on soil that is too acid for healthy plant growth. Powdered limestone (CaCO₃, calcium carbonate) is slower and less effective. All three chemicals react with acids to neutralise them.

You can pre-test the soil with pH paper and match the colour the paper turns with the pH number it indicates. These chemicals can be used on a larger scale in farming and treating acidic rivers and lakes.

The alkali ammonia is a component in some oven cleaners and will react with fatty acids.

Hydrochloric acid and phosphoric acid are components of limescale removers.

Antacid indigestion tablets are mild alkalis that react by neutralising excess stomach acid which is the 'strong' hydrochloric acid which your delicate stomach lining and upper gut can only take so much of! The antacids must be weak (i.e. mild) alkalis as strong alkalis can be just as irritating as strong acids!

Bicarbonate or (sodium hydrogencarbonate NaHCO₃, sodium bicarbonate, baking powder) can be used with sour milk (acidic) for raising action in baking. The acidic milk reacts with 'Bicarb' to form carbon dioxide gas giving the rising action. You can easily demonstrate this by adding any common laboratory acid to baking powder or any other carbonate!

Acidic bee stings (pH 5.0-5.5) can be soothed, i.e. neutralised by calomine lotion, which is a mild alkali and antiseptic and anti-itching agent based on zinc oxide. You can also use baking soda ('bicarb of soda' or sodium hydrogen carbonate), another mild alkali.

Wasp stings are supposed to be alkaline, but apparently not so! they are almost neutral at pH 6.8-6.9 but are 'traditionally' treated with vinegar which is a weak acid (and then perhaps you need the calomine too!). I've come across references on the web to say that wasp stings are not alkaline so 'English folklore' and mild-weak acid treatment has no real scientific basis. It should be pointed out that sting venom is a complex mixture, including many protein-enzymes, which, with other 'foreign' substances, might well trigger a response from the bodies immune system, so, in all honesty, I'm not quite sure what the truth is! However, what is known is that (i) bees and wasps have glands that can secrete either acids or alkalis with other substances and (ii) ants sting venom often contains methanoic acid ('formic acid') which can have a pH of 3 and is presumably 'soothed' by mild alkalis and just to confuse matters more, (iii) many people claim the 'folklore' remedies work! and maybe they do!

In the chemical INDUSTRY

Sodium hydroxide, one of the most commonly used alkalis, is used to neutralise aspirin making 'soluble aspirin'. Aspirin is an organic acid and not very soluble in water, but, its sodium salt is much more soluble and is absorbed faster by the body for more effective treatment.

Ammonia gas is a weak alkali and neutralised by sulphuric acid or nitric acid to form ammonium sulphate or ammonium nitrate salts. These are important agri-chemical fertilisers supplying nitrogen to the soil for better plant growth. Of course some people prefer organic growing using good old muck and compost, but it doesn't involve neutralisation, but it does involve my wife, who is a member of the Soil Association!

Neutralising harmful sulphur dioxide gas (acidic, irritating and toxic SO₂) in power station smoke from burning fossil fuels, by absorbing it in alkaline calcium hydroxide solution (limewater) to absorb it. Eventually harmless calcium sulphate solution is formed.

Acids can be used to clean corroded metal surfaces because of their reactivity to metals and metal oxides to form soluble salts which can be washed away to leave a cleaner metal surface.

So all of this is still pretty important chemistry even for the 21st century, with strong links to agriculture, the environment and leading a stressful life!

Of course there are 'downsides' to some of this 'acidic' chemistry: Acid rain increases the rate of corrosion of stonework (particularly limestone) and metal structures. Acid rain makes water too acid for some aquatic organisms to live and this in turn affects food chains e.g. salmon do not like water with a pH below 4.5! Living on Venus could be hard going, its atmosphere is mainly sulphuric acid, mind you, you should be ok in a plastic suit because plastics don't usually react with acids, which is why, as well as being cheaper, plastics are replacing water pipes, drain pipes and gutters etc. 

• Multiple choice revision quizzes and other worksheets

• GCSE/IGCSE foundation-easier multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE higher-harder multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE Structured question worksheet on Acid Reaction word equations and symbol equation questions

•  Word equation answers and symbol equation answers)

• GCSE/IGCSE word-fill worksheet on Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE matching pair quiz on Acids, Bases, Salts and pH

• See also Advanced Level Chemistry Students Acid-Base Revision Notes - use index

Revision KS4 Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry O Level Chemistry (revise courses equal to US grade 8, grade 9 grade 10) tuition help science chemistry courses revision guides

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